1. Field of the Invention
The present invention relates to the use of fluids for lubricating accessory elements which magnetic tapes come in contact with, during their operation, in the devices containing them.
More in particular, this invention relates to the use of lubricants based on perfluoropolyethers in order to reduce the friction and/or wear phenomena which the magnetic tapes are subjected to owing to the contact with the accessory elements of the cassettes in which the magnetic tapes are contained.
Said friction problems arise in any device utilizing magnetic tapes, however, for the sake of simplicity, the audio or video cassettes will be discussed in particular.
It is understood, however, that the use of the perfluoropolyethers, which is the object of the present invention, can be extended to any device which includes magnetic tapes cassettes, such as, for example, magnetic tapes for computers or typewriters.
2. The Prior Art
As is known, the audio cassettes consist, for example, of an outer casing of plastic material, which consists of two flat valves, which fit into each other and are fixed by means of screws. Inside the valves there are two horizontally arranged reels, round which the magnetic means winds up.
The recording occurs on the tape portion having access to the outside, to which it comes through proper guides and through a window in the cassette edge.
While the device is in operation, the reels are not axially constrained, but are simply caused to rotate by the corresponding pins, so that the tape edge slides against the plastic casing and is subjected to considerable wear and friction phenomena.
During operation, furthermore, the tape comes into contact with other metallic or non-metallic accessory elements, which have the function of guiding the tape so that it can wind up around the respective reels.
To reduce the rubbing of the tape against the plastic casing of the cassette it is known to interpose, between the two parts of plastic material which form the cassette and the reels, two sliding sheets, which are called liners, are generally selflubricating and are obtained by means of punching from a plastic material sheet.
The plastic material generally utilized is based on polyesters, such as e.g. polyethyleneterephthalate.
The interposition of the liners, however, is accompanied by the drawback of leading to a bunching of electrostatic charges by triboelectric effect during the operation of the device, with consequent increase of friciton which results in a worsening of the recording qualities, or jamming of the tape and also, in some cases, breaking of tape.
To overcome these drawbacks it is known to coat the liners surface with a layer of a material containing dispersed graphite. This process consists in depositing on the liner, according to known techniques, a suspension of graphite particles in a suitable binder, generally vinyl resins dissolved in proper solvents. This method is known in the art as graphitized coating.
The graphitized coating is affected however by the drawback of being subjected to progressive wear and flaking during the tape operation due to friction. Furthermore, this method is very expensive as it requires highly sophisticated techniques to obtain very uniform graphite coatings on the liner.
A further complication exhibited by this method is represented by the obtainment of a good adhesion of the graphitized coating to the plastic substrate.
It is also known to utilize lubricants of the class of the polyalcohols; these fluids, however, do not possess good lubricating properties and have a low stability to chemical agents and to oxidation, what limits the performance constancy over rather long stretches of time.
Liners based on graphite-filled polytetrafluoroethylene (PTFE) are known as well. In this case, however, it should be taken into consideration that the liners based on PTFE are very difficult to prepare with the dimensional parameters required for the application in audio cassettes. In fact, PTFE is very difficult to produce in extremely thin films, and this strongly reduces the utilization thereof, as the use of layers of greater dimensions results in too high costs which are not compatible with these uses.
PTFE, though being abstractly a material suited to reduce the friction phenomena, is in practice not utilized because of the above-mentioned drawbacks.
In addition, the sliding on the accessories acting as a guide for the winding up of the tape around the reels during the magnetic tape operation is a further cause of wear and friction.
The sliding phenomenon on accessory elements of the cassette containing the magnetic tape is actually the only cause of friction and wear in the video cassettes, where no sliding sheets are used. In fact, the video cassettes consist of two valves in plastic material and are free of liners, as the magnetic tape is wound up on two reels equipped with flanges, and in order to permit the access to the tape from the outside, the tape is made to pass on two metal cylinders, generally made of stainless steel, and it is kept under tension by a proper tab of plastic material which presses the tape.
As one of the maincharacteristics of the coatings of the tapes' accessory parts is the constancy of their performances over a very long stretch of time, there was a great requirement to find a material capable of reducing to the least possible the friction and wear phenomena as well as the triboelectric effect described hereinbefore.